Since the first implementations of single photon avalanche diodes (SPADs), these devices have established themselves as the detectors of choice in multiple time-correlated imaging methods such as fluorescence lifetime imaging and 3D imaging. For fabricating large arrays of SPADs and to allow for sufficient timing electronics to be integrated on pixel or array level, deep-submicron CMOS technology has been used.
The core of a CMOS SPAD consists of a p-n junction biased above its breakdown voltage, thus operating in Geiger mode. In this regime of operation, free carriers, such as photogenerated electron-hole pairs, can trigger an avalanche breakdown by impact ionization. To avoid premature edge breakdown, a guard ring has been implemented for limiting the electric field at the edges of the junction. SPAD photodiodes have an extremely large gain as a single electron causes a flow of charge (electron-hole pairs) proportional to the full-well capacitance and excess bias voltage of the photodiode. This typically will be in the range of tens to hundreds of thousands. The extremely large gain renders subsequent noise due to electronic processing insignificant. A single digital pulse is emitted for each absorbed photon with a timing accuracy of the order of tens to hundreds of picoseconds. These pulses can be counted or precisely timed to achieve ultra-low light imaging or time-resolved imaging. There are many applications in microscopy, range-sensing, biosensing or biomedical imaging.
Avalanche photodiodes (APDs) operate with the p-n junction biased just below its breakdown voltage. Compared to a Geiger-mode SPAD, an APD provides a finite optical gain, creating multiple electron hole pairs from incident photons. The gain is a nonlinear function of the applied bias voltage. The generated (amplified) photocurrent can be integrated on the APD capacitance to provide a measure of the light intensity as a voltage swing. This voltage suffers from excess noise due to the stochastic nature of the avalanche process.
CMOS image sensors (CIS) commonly employ active pixels composed of a photodiode biased far below breakdown and transistors for charge transfer, buffering, row-addressing and pixel reset. In the CIS case there is no multiplication of the incident light in terms of photo-generated charge; one absorbed photon creates a single electron-hole pair. The generated photocurrent can be integrated on the photodiode capacitance to provide a measure of the light intensity as a voltage swing. CIS is now the dominant technology behind modern digital cameras, having taken over large sectors of the market from CCD.
In other approaches, each of these types of operation, SPAD, APD and CIS, have required specially tailored pixels. There is a need to use the modes of operation of SPAD, APD and CIS in a single imaging application in order to adapt the gain of the photodetector to the intensity of the incoming light to avoid saturation. Therefore, a photodetector and readout circuit which can be reconfigured is desirable in cameras that provide improved cost, simplicity, flexibility and speed of analysis.